The invention relates generally to optical switching elements and more particularly to a system and method for manipulating optical signals within an optical switch.
Communication utilizing optics is increasingly displacing the more traditional electronic or radio wave transmission due to its ability to accommodate a greater rate of transfer per given time. Optical signals are switched in both telecommunication systems and data communication systems. In circuit switching, any one fiber in an array of parallel input optical fibers may be connected to any one fiber in an array of output optical fibers using a matrix of optical switches. As a result, an incoming data packet from a particular input fiber can be directed to a selected output fiber, based upon the destination of the packet.
U.S. Pat. No 4,988,157 to Jackel et al. provides a bi-stable optical switching arrangement utilizing electrochemically generated bubbles. Parallel input waveguides and parallel output waveguides are formed on a substrate at perpendicular angles so as to intersect. A 45xc2x0 slot is formed across each intersection. The slots are selectively filled with a fluid, such as water, or refractive index-matching fluid. Electrodes are positioned adjacent to the slots and are selectively activated to electrolytically convert the fluid to gaseous bubbles. The electrolytic formation of the bubbles destroys any index-matching properties across the slots and causes light to be reflected at the slot sidewall. Thus, an electrolytically formed bubble within a particular slot results in the reflection of optical signals at the slot, rather than the propagation across the slot. The presence of either a catalyst, an electrical pulse of opposite polarity, or an electrical pulse of significant size and of the same polarity will collapse the bubble, thereby returning the switch to a transmissive state.
Although the approach taken by Jackel et al. is simple and potentially inexpensive in large quantities, and achieves a number of advantages over prior approaches, further improvements may be realized. Where water is used as the fluid, electrolysis generates H2 and O2 bubbles in order to create a reflecting state, but the water provides a poor index match to the waveguides. Consequently, crosstalk is high if water is used. Another concern is that the bubble-creation process and the bubble-removal process may be too slow to meet the desired transition time for telecommunication switching. Moreover, the slots are so wide that transmission losses are potentially significant, and sidewalls are so rough that crosstalk is often large.
U.S. Pat. No 5,699,462 to Fouquet et al. provides a more promising approach. The switching arrangement of Fouquet el al. utilizes electrically driven heaters as a means for controlling the direction of the optical signals transversing the switch. Intersecting input waveguides and output waveguides are formed on a silicon substrate. In the transmissive state, an index-matching fluid fills the intersection, enabling light to continue in the input waveguide direction. To initiate switching from the transmissive to the reflective state, a heater is selectively energized to form bubbles within the intersection between the input and output waveguides. The formation of bubbles destroys any index-matching properties across the intersection, resulting in the reflection of optical signals away from the input waveguide direction. A concern with the heater approach is that there is a loss of heat to the surrounding silicon substrate, which necessarily increases the power requirement to create and hold the bubble in place. Another concern is an increase in xe2x80x9cthermal crosstalkxe2x80x9d between the crosspoint at the intersection of the waveguides as the temperature in the surrounding silicon substrate is elevated.
What is needed is a switching element and arrangement which allow reliable transitions between transmitting and diverting states, thereby controlling optical communication between optical signal lines.
A system and method for controlling optical signals in an optical switch utilizes a light-absorbing region or a light-absorbing fluid that is thermally responsive to radiation. Exposing the region or the fluid to radiation allows the manipulation of the fluid within an intersecting gap between a first input optical waveguide and a first output optical waveguide. In one embodiment, the manipulation of the fluid is achieved by selectively projecting radiation onto a material which defines the light-absorbing region and which is located at the intersecting gap. Subjecting the light-absorbing region to the radiation elevates the temperature of the material and therefore the fluid within the gap, resulting in the vaporization of the fluid. Alternatively, the fluid is degassed to form a bubble. Filling the gap with a gas or a bubble creates a refractive index mismatch that causes light from the first input waveguide to be diverted at the intersecting gap, so that the light does not reach the first output waveguide. The diversion is preferably in a direction of a second output optical waveguide. Refilling the gap with fluid switches the propagation of the light from the first input waveguide back to the first output waveguide. In another embodiment, the manipulation of the fluid is achieved by exposing the fluid to the source radiation, where the fluid is highly responsive to the radiation.
Each switching element in an array of such elements may be operated by vaporizing or degassing the fluid to form a small bubble at the intersecting gap for diverting the optical signals from one waveguide to another. In another embodiment, the switching element transfers the fluid from the gap to another location.
In a first embodiment, the light-absorbing region is in thermal contact with the fluid within the gap and includes physical properties for efficiently elevating the temperature of the fluid upon being exposed to the source radiation. The light-absorbing region may be formed by depositing a layer of tungsten, aluminum, copper, iron, cobalt, nickel, tantalum, niobium, zirconium, platinum or molybdenum. Other similar materials may be used. Optionally, the light-absorbing region is fabricated on a substrate (hereinafter xe2x80x9clight-absorbing substratexe2x80x9d) that is subsequently bonded to a waveguide substrate. In an embodiment in which there are two spaced apart light-absorbing regions, the first and the second regions are positioned relative to the gap to devise a push-pull configuration for moving the bubble in order to allow rapid switching between transmission from the first input waveguide to the first output waveguide and reflection from the first input waveguide to the second output waveguide.
In another embodiment, the manipulation of the fluid is achieved by exposing the fluid to the radiation. In this implementation, the fluid is light-absorbing such that subjecting the fluid to the radiation generates thermal energy, resulting in the elevation of the temperature within the gap to form a bubble. The fluid has a narrow absorption spectrum that includes the wavelength characteristic of the source radiation.
Alternatively, a light-absorbing dye is included within the fluid. In this implementation, the fluid is non-light-absorbing. Suitable dyes include metal phthalocyanines, metal naphthalocycanines, or metal-etraphenyl naphthocyanines.
There is a matrix of switching elements for controlling optical communications between input optical waveguides and output optical waveguides that receive signals from the input waveguides at the intersecting gaps. The xe2x80x9cwaveguidesxe2x80x9d may be optical fibers, but are typically multi-layer structures fabricated on a substrate. An acceptable implementation for the waveguides is one in which top and bottom cladded SiO2 waveguides are fabricated on a silica substrate that allows for the propagation of radiation emitted by the optical source. The gap within the silica waveguide substrate (hereinafter xe2x80x9cwaveguide substratexe2x80x9d) may be formed by etching trenches through the waveguides to provide a path for the movement of the fluid toward and away from the gap. Alternatively, liquid flow paths may be etched through the thickness of the waveguide substrate. Other flow paths are also contemplated.
The optical source for generating the source radiation may be any laser diode having the necessary optical intensity to elevate the temperature of the light-absorbing region(s), the light-absorbing fluid, or the light-absorbing dye in the non-light-absorbing fluid so as to manipulate the fluid within the gap. Preferably, the optical source is a Vertical Cavity Surface Emitting Laser (VCSEL). In the embodiment in which there is a matrix of optical switches, there can be a dedicated VCSEL for each optical switch. Alternatively, there is a single VCSEL utilized for switching a number of optical switches. In this implementation, there is a redirecting element dedicated to each optical switch for reflecting the source radiation to the light-absorbing region(s), the light-absorbing fluid, or the light-absorbing dye. The redirecting element may be a mirror or metal coated layer, but this is not critical to the invention. A controller is used to manipulate the displacement of each redirecting element.